Process for preparing a group v111-metal containing catalyst, use thereof for preparing an alkenyl carboxylate

ABSTRACT

A process for preparing a catalyst by  
     (a) selecting a carrier which is a silica based carrier which has been subjected to a series of washings with one or more aqueous liquids consisting of aqueous liquids which have a pH of least 3, when measured at 20° C., or which is a silica based carrier which is formed from materials one or more of which have been subjected to this series of washings,  
     (b) precipitating a Group 8 metal compound onto the carrier,  
     (c) converting the precipitated Group 8 metal compound into metallic species, and  
     (d) subjecting the Group 8 metal/carrier composition to a purification treatment, before or after step (c); a catalyst which is obtainable by this process; and a process for preparing an alkenyl carboxylate by reacting a mixture comprising an olefin, a carboxylic acid and oxygen in the presence of the catalyst.

FIELD OF THE INVENTION

[0001] The invention relates to a process for preparing a catalyst, to acatalyst which is obtainable by the process of this invention, and to aprocess for preparing an alkenyl carboxylate comprising reacting amixture comprising an olefin, a carboxylic acid and oxygen in thepresence of the catalyst.

BACKGROUND OF THE INVENTION

[0002] Catalysts for the preparation of an alkenyl carboxylate from anolefin, a carboxylic acid and oxygen are known in the art. Suchcatalysts are based on a Group 8 metal as a catalytically activemetallic species on a carrier. The preparation of the catalysts is welldocumented.

[0003] For example, the process for preparing the catalyst of U.S. Pat.No. 4,048,096 comprises the steps of selecting a carrier, precipitatinga Group 8 metal compound into the carrier, converting the precipitatedGroup 8 metal compound into metallic species, and subsequently purifyingthe catalyst by washing with water.

[0004] Similar schemes for the preparation of the catalysts are knownfrom U.S. Pat. No. 5,179,057 and U.S. Pat. No. 5,189,004. The latterdocuments highlight the removal from the catalysts of sodium ions, whichare introduced during the catalyst preparation, for example as a portionof a precursor of the precipitated Group 8 metal compound (for examplesodium tetrachloropalladium (II)), or as a portion of the precipitatingagent which is used for precipitating the Group 8 metal compound (forexample a sodium silicate or sodium hydroxide). Both documents teachthat the removal of sodium ions leads to an increase in the activity ofthe catalyst and to a decrease in the selectivity when the catalyst isused in the process for the preparation of an alkenyl carboxylate.

[0005] U.S. Pat. No. 5,250,487 and U.S. Pat. No. 5,422,329 relate toGroup 8 metal catalysts for use in a process for the preparation of analkenyl carboxylate from an olefin, a carboxylic acid and oxygen. Thecatalysts are based on carrier particles which have been pressed withthe aid of a binder of one or more salts of carboxylic acids. Thecarrier particles are washed with an acid for the removal of the cationsof the binder from the support particles. The acid may be a mineral acidsuch as hydrochloric acid, sulfuric acid, phosphoric acid, or nitricacid. If the anion of the acid is detrimental to the catalyst, such aschloride and sulfate anions, excess acid may be removed by washing outwith distilled water. If the salts employed during the subsequentcatalyst preparation contain constituents harmful to the catalyst, suchas chloride or sulfate, the catalyst is washed with water.

[0006] Thus, in the prior art documents relating to the Group 8 metalcatalysts attention has been paid to the detrimental effects of certaincatalyst impurities which are introduced intentionally during thecarrier or catalyst preparation and to the removal of these impurities.Such impurities may be removed by using dedicated means. For example,the cations of the binder are removed by washing with acid.

[0007] WO-00/15333 teaches the preparation of improved catalysts, inparticular silver impregnated alumina based catalysts for the vaporphase production of epoxides. The improvement is achieved by loweringthe concentration of ionizable species present on the surface of thecarrier, in particular by treating the carrier by washing with boilingwater prior to catalyst preparation.

[0008] Although the known Group 8 metal catalyst have appreciableactivity and selectivity in the preparation of alkenyl carboxylate froman olefin, a carboxylic acid and oxygen, further improvements of thesecatalysts are desirable, in particular in their ability to maintaintheir level of activity and selectivity during use over a long period oftime.

SUMMARY OF THE INVENTION

[0009] It has now unexpectedly been found that the Group 8 metalcatalysts having an improved catalyst performance in the preparation ofalkenyl carboxylates are prepared if prior to the catalyst preparationthe carrier is subjected to washing with water, even though waterwashing is also applied in the course of the catalyst preparation.

[0010] By the term “improved catalyst performance” it is meant thatthere is an improvement in at least one of the catalyst properties,which catalyst properties include catalyst activity, selectivity,activity or selectivity performance over time, operability (i.e.resistance to run-away), conversion and work rate. By “selectivity” itis meant the selectivity to alkenyl carboxylate, based on the quantityof olefin converted. The improvement in question concerns in particularthe ability of the catalyst to maintain its level of activity andselectivity during use over a long period of time.

[0011] This result is unexpected in view of the prior teachings whichrelate to the Group 8 metal catalysts, as referred to hereinbefore.Namely, one skilled in the art would expect that the washings in thecourse of the catalyst preparation, as taught by these documents, wouldlead also to the removal of impurities already present in the carrierper se, so that no further advantageous effect could be expected from anadditional washing of the carrier, i.e. prior to the catalystpreparation.

[0012] The beneficial effect of subjecting the carrier to washing withwater, in addition to washing in the course of the catalyst preparationis also unexpected in view of WO-00/15333, referred to hereinbefore.Namely, WO-00/15333 does not teach any subsequent washing, i.e. as astep of the catalyst preparation or subsequent to the catalystpreparation.

[0013] Accordingly, the invention provides a process for preparing acatalyst which process comprises the steps of

[0014] (a) selecting a carrier which is a silica based carrier which hasbeen subjected to a series of washings with one or more aqueous liquidsconsisting of aqueous liquids which have a pH of least 3, when measuredat 20° C., or which is a silica based carrier which is formed frommaterials one or more of which have been subjected to a said series ofwashings,

[0015] (b) precipitating a Group 8 metal compound onto the carrier,

[0016] (c) converting the precipitated Group 8 metal compound intometallic species, and

[0017] (d) subjecting the Group 8 metal/carrier composition to apurification treatment, before or after step (c).

[0018] The invention also provides a process for preparing a catalystwhich process comprises the steps of

[0019] (a) washing a carrier with one or more aqueous liquids consistingof aqueous liquids which have a pH of least 3, when measured at 20° C.,wherein the carrier is a silica based carrier,

[0020] (b) precipitating a Group 8 metal compound onto the carrier,

[0021] (c) converting the precipitated Group 8 metal compound intometallic species, and

[0022] (d) subjecting the Group 8 metal/carrier composition to apurification treatment, before or after step (c).

[0023] The invention also provides a catalyst which is obtainable by aprocess for preparing a catalyst according to this invention.

[0024] The invention also provides a process for preparing an alkenylcarboxylate comprising reacting a mixture comprising an olefin, acarboxylic acid and oxygen in the presence of the catalyst of thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0025] The silica based carrier for use in this invention may be of anykind. For example, the carrier may comprise further materials such asalumina, magnesia, zirconia, fuller's earth, artificial and naturalzeolites, and combinations thereof, in particular alumina. The silicacontent of the carrier is typically at least 50% w, more typically atleast 90% w, based on the weight of the carrier. Frequently the silicacontent of the carrier is at most 99.99% w, more frequently at most99.9% w, on the same basis.

[0026] Typically, the carrier is a porous carrier, preferably having aspecific surface area of at least 0.01 m²/g, in particular in the rangeof from 0.05 to 1000 m²/g, more in particular in the range of from 0.2to 1000 m²/g, as measured by the B.E.T. method, and a water absorptioncapacity of from 0.05 to 3 ml/g, in particular from 0.1 to 2 ml/g, asmeasured by conventional water absorption technique. The B.E.T. methodas referred to herein has been described in detail in S. Brunauer, P. Y.Emmett and E. Teller, J. Am. Chem. Soc. 60, 309-16 (1938).

[0027] Of particular interest are silicas which have a specific surfacearea in the range of from 10 to 1000 m²/g, in particular from 50 to 500m²/g, as measured by the B.E.T. method.

[0028] Regardless of the carrier used, it may be shaped into particles,chunks, pieces, and the like. Preferably, for use in a tubular fixed bedreactor, they are formed into a rounded shape, for example in the formof spheres, pellets, cylinders, rings or tablets, typically havingdimensions in the range of from 2 mm to 2 cm.

[0029] For use in this invention, the carrier is subjected to a seriesof washings with one or more aqueous liquids. A series of washings isherein understood to include a single washing step and a combination ofconsecutive washing steps which employ one or more washing liquids. Inaccordance with this invention the washing liquids comprise aqueousliquids which all have a pH of at least 3, when measured at 20° C. Oneskilled in the art will appreciate that aqueous liquids may contain asmall quantity of acid, such as resulting from dissolution ofatmospheric carbon dioxide, and they may therefore have a pH slightlybelow 7, for example down to a pH of 3. Such aqueous liquids containvery little acid or the acid is a weak acid, so that yet they areconsidered to be essentially non-acidic aqueous liquids.

[0030] Preferably, the aqueous liquids have all a pH of at least 5, inparticular at least 6, more in particular at least 7, when measured at20° C. Typically, the washing liquids have all a pH of at most 10, inparticular at most 9, more in particular at most 8, when measured at 20°C.

[0031] As used herein, the term “pH” refers to the pH of an aqueousliquid as measured by using a conventional pH measuring probe which iscalibrated by using buffer solutions.

[0032] Eligibly, the aqueous liquids comprise for the greater partwater, and they may or may not comprise relatively small quantities ofother components, for example organic materials, for example esters,ethers, alcohols or ketones, or salts like acetates, carbonates,nitrates or oxalates, in particular such salts as lithium, sodium,potassium, ammonium, monoalkylammonium, dialkylammonium,trialkyl-ammonium and tetraalkylammonium salts. Such other componentsmay not be detrimental to the preparation of the catalyst or to theperformance of the catalyst when used in the preparation of alkenylcarboxylates. Otherwise, such other components, when left behind on thecarrier after the washing may be removed from the carrier, for exampleby further washing the carrier, by evaporation or by decomposition (i.e.by calcination).

[0033] Not wishing to be bound by theory it is believed that as a resultof the washing ionizable species are removed from the carrier, or atleast from the carrier surface, which ionizable species have aninfluence on the precipitation and/or the conversion into metallicspecies, such that the morphology of the active surface of the catalystis changed to an extent which favors the catalyst performance in thepreparation of alkenyl carboxylates. It is believed that the followingionizable species may be associated with these effects: silicates,aluminosilicates, sulfates, chlorides, sodium salts, aluminium salts,calcium salts, magnesium salts, and the like.

[0034] The aqueous liquids which comprise a salt as specifiedhereinbefore may be called ion exchange solutions. The presence of saltsin the aqueous liquids may facilitate the removal of ionizable specieswhich are firmly bound to the carrier, so that a desirable result may beachieved in a shorter time or at a lower temperature.

[0035] Suitably, the ion exchange solution comprises the salt in aquantity of at most 0.1 moles/l. Suitably, the ion exchange solutioncomprises the salt in a quantity of at least 0.001 moles/l. Preferably,the ion exchange solution comprises the salt in a quantity in the rangeof from 0.002 to 0.05 moles/l. The remainder of the solution may be ade-ionized liquid as specified hereinafter.

[0036] The water content of the aqueous liquids, in particular when theydo not comprise added salt as to form an ion exchange solution, ispreferably at least 90% w, more preferably at least 99% w, in particularat least 99.9% w, more in particular at least 99.99% w, relative to theweight of the aqueous liquid. Frequently, the content of water is atmost 99.999% w, on the same basis. Preferably, the aqueous liquid iswater.

[0037] Suitably, the aqueous liquids have a low conductivity. Suchlow-conductivity aqueous liquids typically do not comprise added salt asto form an ion exchange solution. Suitably, the conductivity is at most500 μmho (mho is Ω⁻¹, or Siemen, or S), more suitably at most 100 μmho,preferably at most 20 μmho, in particular at most 5 μmho, when measuredat 98° C. Frequently the conductivity will be at least 0.1 μmho, morefrequently at least 0.2 μmho, on the same basis. Conductivities areherein understood to be electrical conductivities, measured by using aconductivity measuring probe having a cell constant of 1.0/cm. Suitablya YSI Model 3401 (trademark) conductivity measuring probe is used,connected to a YSI Model 35 (trade mark) conductance meter. Suchlow-conductivity aqueous liquids are typically de-ionized aqueousliquids. The de-ionized aqueous liquids are obtainable by de-ionisationusing an ion exchange material such as an ion exchange resin, typicallya cation exchange material in the acidic form, or an anion exchangematerial in the basis form, but preferably using a cation exchangematerial in the acidic (H⁺) form and an anion exchange material in thebasic (OH⁻) form.

[0038] The extent and the type of washing are not material to thepresent invention. The washing may be carried out in a continuousfashion or it may be a batch type operation. There may be one washing,but the number of washings may also be two, or three, or more, forexample up to five or ten. The quantity of aqueous liquid used in thewashings relative to the quantity of the carrier is also not material tothe invention. The washing may be carried out a temperature in the rangeof from 10 to 300° C., preferably at a temperature in the range of from50 to 150° C., for example about 100° C. However, when using an ionexchange solution, the temperature is preferably in the range of from 20to 120° C., for example about 70° C.

[0039] The washing may be monitored by applying a conductivity test,which conductivity test involves contacting samples of water withsamples of the carrier, unwashed and washed, and measuring theconductivity of each water sample after it has reached equilibrium withthe respective carrier sample at 95° C. In this conductivity test theconductivity is measured at 95° C., the quantity of water sample is 3g/g carrier sample and the conductivity of the water prior to thecontacting with the carrier sample is 1.5 μmho at 98° C. Water eligiblefor use in the conductivity test is water which is de-ionized using acation exchange material in the H⁺ form and an anion exchange materialin the OH⁻ form.

[0040] Typically the carrier is washed to the extent that in theconductivity test the conductivity measured for the washed carrier isless than 50% of the value found for the unwashed catalyst, preferablyless than 30%, more preferably less than 20%. Frequently, the carrier iswashed to the extent that in the conductivity test the measuredconductivity is at least 1%, more frequently at least 5% of the valuefound for the unwashed catalyst.

[0041] Typically the carrier is washed to the extent that in theconductivity test the conductivity measured for the washed carrier isless than 200 μmho, more typically less than 75 μmho, preferably lessthan 50 μmho. The carrier may be washed such as to achieve that theconductivity measured in the conductivity test is as low as possible.However, in practice the carrier may be washed to the extent that theconductivity measured in the conductivity test is above 2 μmho, morefrequently above 3 μmho.

[0042] As an alternative to, or in addition to washing the carrier, oneor more materials from which the carrier is formed may be subjected to aseries of washing as, and to the extent described hereinbefore.Subsequently, the carrier may be formed from the materials byconventional mixing and/or shaping methods, such as extrusion.

[0043] The Group 8 metal for use in this invention has suitably anatomic number of at least 44 and at most 78. One or more Group 8 metalsmay be applied. Preferably, the Group 8 metal is palladium.

[0044] Preferably, the catalyst is based on a Group 1b metal, inaddition to a Group 8 metal. One or more Group 1b metals may be applied.The Group 1b metal is preferably gold.

[0045] The terms “Group 8 metal” and “Group 1b metal” as used hereinrefer to the metals of Group 8 and Group 1b, respectively, of thePeriodic Table of the Elements as published in R C Weast (Ed,) “Handbookof Chemistry and Physics”, 54^(th) edition, CRC Press, inside cover.

[0046] In a preferred embodiment, the catalyst is based on palladium asthe Group 8 metal and gold as the Group 1b metal.

[0047] The term “Group 8 metal/carrier composition”, as used hereinrefers to any composition comprising the carrier and a Group 8 metaldispersed on the carrier, irrespective of whether the Group 8 metal ispresent as a Group 8 metal compound or Group 8 metal compound precursor,or in the form of metallic species.

[0048] Suitably the Group 8 metal compound and optionally the Group 1bmetal compound is precipitated onto the carrier by pore impregnating thecarrier with one or more aqueous solutions comprising a Group 8 metalcompound precursor and optionally a Group 1b metal compound precursorand then precipitating the Group 8 metal compound and optionally theGroup 1b metal compound onto the carrier from such solutions, by using aprecipitating agent. In more detail, the applicable materials andmethods may be those as disclosed in U.S. Pat. No. 4,048,096, U.S. Pat.No. 5,179,057 and U. S. Pat. No. 5,189,004, which are incorporatedherein by reference.

[0049] The volume of an impregnation solution preferably corresponds toat least 80%, preferably 95 to 100% of the water absorption capacity ofthe carrier.

[0050] Eligible Group 8 metal compound precursors and Group 1b metalcompound precursors are for example water soluble acids and salts, suchas chlorides, nitrates, nitrites and sulfates. Preferred such Group 8containing acids and salts are palladium (II) chloride, palladium (II)nitrate, and palladium (II) sulfate and, in particular, sodium palladium(II) tetrachloride. Preferred such Group 1b metal containing acids andsalts are auric (III) chloride and, in particular, tetrachloroauric(III) acid.

[0051] The precipitating agent includes for example alkali metalhydroxides, alkali metal bicarbonates, alkali metal carbonates and,preferably, alkali metal silicates. Suitable alkali metals are lithium,sodium and potassium. The preferred precipitating agent is sodiumsilicate. A useful form of sodium silicate is sodium metasilicatepentahydrate. The precipitating agent is suitably used in an excessrelative to the Group 8 metal and optionally the Group 1b metal, takentogether. For example, the precipitating agent may be used in a quantityof 1 to 3 moles, preferably 1.5 to 2.5 moles per mole of Group 8 metalcompound precursor. If the Group 1b metal precursor is present anadditional quantity of the precipitating agent may be used, for example2 to 10 moles, preferably 2.5 to 8 moles per mole of Group 1b metalcompound precursor.

[0052] The precipitating agent is preferably used as an aqueoussolution. The aqueous solution has typically a volume sufficient tocover the impregnated, wet carrier particles. As an alternative, afterthe impregnation with one or more solutions comprising a Group 8 metalcompound precursor and optionally a Group 1b metal compound precursor,the carrier particles may be dried and subsequently impregnated with anaqueous solution comprising the precipitating agent. In the latter case,the volume of the aqueous solution of the precipitating agent typicallycorresponds to at least 80%, preferably 95 to 100% of the waterabsorption capacity of the carrier.

[0053] The temperature at which the precipitation may be carried out istypically in the range of from 1 to 100° C., more typically in the rangeof from 5 to 50° C., for example about 20° C. The reaction time appliedin the precipitation step may be for example at least 2 hours, morepreferably at least 3 hours, and it may be for example up to 100 hours,more typically it is in the range of from 6 to 40 hours, for example 24hours. During the precipitation, the particles may be left static, orthey may be moved relative to the solution of the precipitation agent,or relative to each other. For example, the particles may be movedrelatively to each other during the initial stages of the precipitation,for example during the first 15 minutes, or first 30 minutes, or firsthour. Upon completion of the precipitation, the pH of the precipitatingsolution is preferably in the range of from 6.5 to 11, for example 6.5to 9.5, but more preferably it is in the range of from 7.5 to 10, inparticular from 7.5 to 8, when measured at 20° C. The final pH may beadjusted by changing the amount of the precipitating agent.

[0054] The quantity of the Group 8 metal compound precursor may be suchthat in the catalyst as prepared the content of the Group 8 metal istypically in the range of from 10 to 500 mmoles/kg catalyst, andpreferably in the range of from 20 to 200 mmoles/kg catalyst, forexample about 75 mmoles/kg or about 138 mmoles/kg.

[0055] The quantity of the Group 1b metal compound precursor may be suchthat in the catalyst as prepared the content of the Group 1b metal istypically in the range of from 1 to 200 mmoles/kg catalyst, andpreferably in the range of from 5 to 100 mmoles/kg catalyst, for exampleabout 37.2 mmoles/kg or about 65 mmoles/kg.

[0056] The precipitated Group 8 metal compound and, if present, theGroup 1b metal compound may be converted into metallic species. If theGroup 8 metal in the Group 8 metal compound and, if present, the Group1b metal in the Group 1b metal compound are not in their zero valencestate, the conversion into metallic species may be accomplished byreduction. Suitable reducing agent and reduction methods are known fromU.S. Pat. No. 4,048,096 , U.S. Pat. No. 5,179,057 and U.S. Pat. No.5,189,004, which are incorporated herein by reference.

[0057] For example, the reduction may be accomplished by using as areducing agent diborane; amines, such as ammonia and hydrazine;carboxylic acids and their salts, such as oxalic acid, potassiumoxalate, formic acid, potassium formate, ammonium citrate; aldehydes,such as formaldehyde, acetaldehyde; hydrogen peroxide; reducing sugarssuch as glucose; alcohols other than reducing sugars, such as methanoland ethanol; polyhydric phenols, such as hydroquinone and catechol;hydrogen; carbon monoxide; olefins, such as ethylene, propene andisobutene; or sodium borohydride. The reduction may be carried out in anaqueous solution which comprises for example at most 50 mole, preferablyat most 25 mole of the reducing agent per mole of Group 8 metal presentin the Group 8 metal compound and, if present, the Group 1b metal in theGroup 1b metal compound. This will frequently result in a completereduction of the Group 8 metal and, if present, the Group 1b metal.Suitably, so much of the reducing agent is used which is sufficient tocomplete the reduction of at least 90%, more suitably at least 99% ofthe Group 8 metal and, if present, the Group 1b metal to metallicspecies.

[0058] Preferably, hydrogen is employed as the reducing agent. Whenhydrogen is employed, typically no liquid diluent is present, typicallythe absolute pressure is in the range of from 50 to 2000 kPa (0.5 to 20bar), more typically from 100 to 1000 kPa (1 to 10 bar), typically thehydrogen partial pressure is in the range of from 1 to 2000 kPa (0.01 to20 bar), and typically the temperature is in the range of from 10 to300° C., more typically from 50 to 250° C.

[0059] In another preferred embodiment, hydrazine is employed as thereducing agent. When hydrazine is employed, typically an aqueous diluentis present, and typically the temperature is in the range of from 0 to100° C., more typically from 5 to 50° C., for example 20° C.

[0060] Combinations of reducing agents may be used, or two or moreseparate reduction steps may be applied. For example, the precipitatedGroup 8 metal compound and, if present, the precipitated Group 1b metalcompound may be reduced in part with a first reducing agent, for exampleabout 20%-mole or about 40%-mole or about 60%-mole of the total of Group8 metal and Group 1b metal (if any), and in a subsequent step, theremaining part may be reduced with a second reducing agent. The firstand second reducing agents may independently be selected from thereducing agents described hereinbefore.

[0061] Preferred first reducing agents are selected from diborane;amines, such as ammonia and hydrazine; carboxylic acids and their salts,such as oxalic acid, potassium oxalate, formic acid, potassium formate,ammonium citrate; aldehydes, such as formaldehyde, acetaldehyde;hydrogen peroxide; reducing sugars such as glucose; polyhydric phenols,such as hydroquinone and catechol; or sodium borohydride. The preferredfirst reducing agent is hydrazine.

[0062] The second reducing agents may suitably be selected fromhydrogen; carbon monoxide; alcohols, such as methanol and ethanol;aldehydes, such as formaldehyde and acetaldehyde; and olefins, such asethylene, propene and isobutene. The preferred second reducing agent ishydrogen.

[0063] The reduction in which the first reducing agent is employed ispreferably carried out as a liquid phase reaction, i.e. a reductionwhich involves contacting the Group 8 metal/carrier composition with thefirst reducing agent present in a liquid phase. The reduction in whichthe second reducing agent is employed is preferably carried out as a gasphase reaction, i.e. a reduction which involves contacting the Group 8metal/carrier composition with the second reducing agent present in agas phase, in the absence of a continuous liquid phase, preferably inthe absence of a liquid phase.

[0064] In a particular embodiment, a major amount of the precipitatedGroup 8 metal compound and, if present, the precipitated Group 1b metalcompound may be reduced in the liquid phase reaction by using the firstreducing agent. This may be achieved by employing a suitable amount ofthe first reducing agent and allowing the first reducing agent tocompletely react away. More of the first reducing agent may be needed ifit has a tendency to decompose, under the prevailing reactionconditions. The major amount may be at least 50%-mole, and preferably itmay represent from 70 to 99%-mole, more preferably from 80 to 90%-mole,of the total of the Group 8 metal and Group 1b metal (if any). In thisparticular embodiment, the reduction with the first reducing agent maybe followed by the purification step (c), subsequently optionally by adrying step as described hereinafter, and thereafter the remaining partof the Group 8 metal and Group 1b metal (if any) may be reduced with thesecond reducing agent, in particular, hydrogen. This sequence of stepsis advantageous as problems are avoided which are associated with thedisposal of excess, unconverted reducing agents (these may be noxiousmaterials, such as hydrazine), while it minimizes possible losses ofGroup 8 metal and Group 1b metal (if present) during the purificationstep (c), and it leads to a high yield of metallic species on thecatalyst. Further, it has unexpectedly been found that by applying thecombination of a liquid phase reaction and a gas phase reaction insteadof applying the liquid phase reaction only, there is an improvement inthe dispersity of the metallic species in the catalyst, as measured byan increased carbon monoxide chemisorption of the catalyst. It istheorized that a better dispersity of the metallic species leads to animproved catalyst performance.

[0065] In another embodiment the Group 8 metal compound precursor and,if present, optionally the Group 1b metal compound precursor isprecipitated and converted into metallic species in one step, followingfor example procedures as disclosed in WO-99/08790 and WO-99/08791. Thismeans that in the catalyst preparation according to this invention step(b) and step (c) may be carried out as a single step.

[0066] The purification treatment suitably involves a series of washingsof the Group 8 metal/carrier composition, with one or more aqueousliquids, with the objective of removing at least a portion of theuseless chemicals, which are left on the Group 8 metal/carriercomposition after accomplishing the precipitation of the Group 8 metalcompound on the carrier, and, optionally, after converting the Group 8metal compound into metallic species.

[0067] Eligibly, the aqueous liquids for use in the purificationtreatment may be selected from the aqueous liquids as specifiedhereinbefore for the washing of the carrier of steps (a). Thepurification treatment may be carried out at a temperature in the rangeof from 0 to 100° C., preferably at a temperature in the range of from 5to 50° C., for example about 20° C.

[0068] The purification treatment may be monitored by any suitablemeans, for example by applying the conductivity test as describedhereinbefore. Alternatively, the purification treatment may be monitoredby following the disappearance of contaminants which are to be removed,such as sodium ions or chloride, in accordance with the nature of, forexample, the Group 8 metal compound precursor, the precipitating agentand the reducing agent. In this respect, reference may be made to U.S.Pat. No. 4,048,096, U.S. Pat. No. 5,179,057 and U.S. Pat. No. 5,189,004,which are incorporated herein by reference.

[0069] The purification treatment, i.e. step (d), may be carried outafter step (c) and before an optional step (e), as describedhereinafter, or after step (b) and before step (c). As set outhereinbefore, it may be advantageous to carry out a step (c) before step(d) and to carry out another step (c) after step (d), and suitablybefore an optional step (e).

[0070] The present process for preparing a catalyst may comprise inaddition the step of (e) impregnating with a source of an alkali metal,such as disclosed in U.S. Pat. No. 4,048,096, U.S. Pat. No. 5,179,057and U.S. Pat. No. 5,189,004, which are herein incorporated by reference.Any source of an alkali metal may be used such that the alkali metaldeposited on the Group 8 metal/carrier composition can form an alkalimetal carboxylate or maintain the presence of an alkali metalcarboxylate upon contact with an carboxylic acid, such as during thesubsequent use of the catalyst in the preparation of an alkenylcarboxylate.

[0071] Suitable sources of an alkali metal are for example alkali metalcarbonates and, preferably, alkali metal carboxylates. The alkali metalcarboxylate is typically derived from a mono carboxylic acid, such asbutyric acid, propionic acid and, preferably, acetic acid. The alkalimetal may be any one or more of lithium, sodium, potassium, rubidium andcesium. Preferably, the alkali metal is potassium. The preferred alkalimetal carboxylate is potassium acetate. The quantity of the alkali metalcarboxylate is typically such that the alkali metal content of thecatalyst is in the range of from 0.1 to 5 mole/kg, more preferably from0.2 to 2 mole/kg catalyst, for example 340 mmole/kg, or 585 mmole/kg, or765 mmole/kg, or 1560 mmole/kg.

[0072] The step of impregnating with an alkali metal carboxylate may becarried out at any stage of the catalyst preparation. Preferably, theGroup 8 metal/carrier composition is impregnating with an alkali metalcarboxylate after step (d), in particular after step (d) and step (c).

[0073] At certain stages of the catalyst preparation it may be desirableto perform a drying step. Drying is typically performed at a temperaturein the range of from 50 to 300° C., more typically in the range of from80 to 150° C., for example 90° C., or 115° C., or 120° C., using aninert gas, such as nitrogen or helium, or air.

[0074] A drying step may be carried out, for example, following theinitial carrier washing (cf. steps (a)), following the purificationtreatment (cf. step (d)) or following the impregnation of step (e).Preferably, the last step in the catalyst preparation in which liquidsare involved is a drying step, after which the catalyst may or may notbe subjected to essentially dry operations, such as milling and sieving.

[0075] The catalyst which is prepared by the methods describedhereinbefore is typically a shell type catalyst, i.e. a catalyst whichcomprises the catalytically active species, i.e. the Group 8 metallicspecies, in the surface layer of the carrier. For example, 90 mole-% ofthe Group 8 metallic species may be distributed within the surface layerextending at most 2 mm from the surface of the carrier. In morepreferred embodiments, 90 mole-% of the Group 8 metallic species may bedistributed within the surface layer extending at most 1.5 mm, inparticular at most 1 mm, from the surface of the carrier. Frequently thesurface layer in question extends at least 0.05 mm, in particular atleast 0.1 mm, from the surface of the carrier.

[0076] The present process for preparing an alkenyl carboxylatecomprises reacting a mixture comprising an olefin, a carboxylic acid andoxygen in the presence of the catalyst of this invention. The process isfrequently a gas phase process, wherein a gaseous feed comprising thereactants is contacted with the solid catalyst. The catalyst is suitablypresent in the form of a fluidized bed of catalyst particles, or, moresuitable, in the form of a packed bed. The process may be carried out asa batch process, however, it is more suitable to carry out the processas a continuous process.

[0077] The carboxylic acid is preferably a monocarboxylic acid, forexample butyric acid, propionic acid, or preferably acetic acid.

[0078] The olefin is typically a monoolefin, for example 1-butene,2-butene, isobutene, propylene, or preferably ethylene.

[0079] Most preferably, the carboxylic acid is acetic acid and theolefin is ethylene, in which case the alkenyl carboxylate is vinylacetate.

[0080] The quantity of carboxylic acid is suitably in the range of from1 to 20%-mole, more suitably in the range of from 5 to 15%-mole,relative to the number of moles of the feed. The quantity of olefin issuitably in the range of from 10 to 80%-mole, more suitably in the rangeof from 30 to 60%-mole, relative to the number of moles of the feed. Thequantity of oxygen is suitably in the range of from 1 to 15%-mole, moresuitably in the range of from 5 to 10%-mole, relative to the number ofmoles of the feed. One skilled in the art will understand that for agaseous feed a mole fraction corresponds with a volume fraction.

[0081] The source of oxygen may be air. Air may be used in the processof this invention, but it is preferred that an oxygen containing gaswhich may be obtained by separation from air is used.

[0082] Furthermore, inert compounds may be present in the mixture, forexample methane, ethane, carbon dioxide, nitrogen or argon. Inertcompounds may typically be present in a quantity of from 5 to 80%-mole,more typically from 10 to 60%-mole, relative to the number of moles ofthe feed.

[0083] The process may preferably be carried out at a temperature in therange of from 100 to 250° C., in particular in the range of from 130 to200° C. As time proceeds, the temperature may be increased gradually, asto compensate for loss in activity of the catalyst, if any. The processmay preferably be carried out at a pressure in the range of from 1 to 25barg (i.e. bar gauge), in particular in the range of from 1 to 20 barg.

[0084] In general, it is preferred to operate at a high oxygenconcentration. However, in actual practice in order to remain outsidethe flammability limits of the reactor streams, the concentration ofoxygen has to be lowered as the concentration of the olefin and/oroxygenate is increased. The actual safe operating conditions dependalong with the gas composition, also on individual plant conditions,such as temperature and pressure. Therefore, for each individual plantthe concentration of oxygen will be determined which may be used withany concentration of the olefin and the oxygenate.

[0085] When operating the process as a gas phase process using a packedbed reactor, the GHSV may preferably be in the range of from 1000 to10000 Nl/(l.h). The term “GHSV” stands for the Gas Hourly SpaceVelocity, which is the volumetric flow rate of the feed, which is hereindefined at normal conditions (i.e. 0° C. and 100 kPa (1 bar) absolute),divided by the volume of the catalyst bed.

[0086] The alkenyl carboxylate may be recovered from the reactionproduct by known means, such as by fractional distillation or reactivedistillation.

[0087] Unless specified otherwise, the organic compounds mentionedherein have typically at most 10 carbon atoms, in particular at most 6carbon atoms. Organic compounds are deemed to be compounds whichcomprise carbon atoms and hydrogen atoms and carbon-hydrogen bonds intheir molecules.

[0088] It is apparent that certain features of the invention, which arefor clarity described herein in the context of separate embodiments, mayalso be provided in combination in a single embodiment. Conversely,features of the invention which are described in the context of a singleembodiment may also be provided separately or in any suitablesub-combination.

[0089] The invention will be illustrated by means of the following,non-limiting examples.

EXAMPLE 1

[0090] (For Comparison)

[0091] A catalyst was prepared by applying the following steps:

[0092] 1. A 25 g sample of a silica spheres carrier (spheres diameter 5mm, surface area 137 m²/g, water absorption capacity 0.63 ml/g, obtainedfrom Südchemie, under the trademark KA-160), was dried at 120° C. inair, cooled down to ambient temperature in a dessicator and subsequentlyimpregnated with 15.7 ml of a solution of sodium palladium (II)tetrachloride (III) (Na₂PdCl₄) and tetrachloroauric acid (HAuCl₄) inde-ionized water containing 0.220 g palladium and 0.121 g gold. Acontainer holding the carrier was gently shaken to allow solution uptakeby the carrier. After complete uptake of the solution the impregnatedcarrier was allowed to stand for 2 hours at room temperature.

[0093] 2. Then 30 ml of a solution containing 1.68 g sodium metasilicatepentahydrate (Na₂SiO₃.5H₂O) was added to completely cover the wetimpregnated support. This was allowed to stand for 15 hours.

[0094]3. Subsequently, 1 ml of 85% w hydrazine hydrate was added, mixedgently and allowed to stand for 4 hours at room temperature to reducethe palladium and gold salts to metallic species.

[0095] 4. The palladium/carrier composition was then washed withdistilled de-ionized water three times by decantation followed by acontinuous wash until the wash water was free of chloride, as checked bythe absence of a precipitate when tested with a silver nitrate solution.The washed palladium/carrier composition was then dried at about 120° C.for 4 hours under nitrogen, and cooled in a container protected frommoisture

[0096] 5. The palladium/carrier composition was then impregnated with15.7 ml of a solution of potassium acetate in de-ionized watercontaining 1.34 g potassium, dried at about 120° C. for 15 hours undernitrogen, and cooled.

[0097] The catalyst thus prepared was tested in a reaction tube having alength of 30 cm and an inside diameter of 1.51 cm. The tube was loadedwith 2.5 g of catalyst diluted into a 10 cm bed of glass beads. Thereaction tube was fed with a gaseous mixture of 49 mole-% ethylene, 13mole-% acetic acid and 7.6 mole-% oxygen (balance nitrogen). The GHSVwas 4250 Nl/(l.h), calculated on undiluted catalyst. With the catalysttemperature initially at 147° C., and the absolute pressure at 880 kPa(i.e. 7.8 barg), the space-time-yield was 750 g vinyl acetate/(lcatalyst.h), and the initial selectivity was 93%. The jacket temperaturewas increased slowly so as to keep the space-time-yield constant at 750g vinyl acetate/(l catalyst.h). After 425 hours, the jacket temperaturewas 190° C., and the selectivity was 84%.

EXAMPLE 2

[0098] (According to the Invention)

[0099] The procedures of Example 1 was repeated, except that the sampleof silica spheres carrier was subjected to washing before use in step 1.This washing was carried out by immersing a 500 g sample of the carrierin boiling de-ionized water (6 kg, having a conductivity of 1.5 μmho at98° C.) in a continuously replenishing vessel (flow rate 0.76 l/min).The conductivity of the washing water was measured continuously at 95°C. After 12 minutes the peak value of the conductivity was 60 μmho andafter 120 minutes the conductivity was 6 μmho, at which point thecarrier was subjected to the drying of step 1.

[0100] At any point in time the washing water may be deemed to havereached equilibrium with the carrier. The conductivity data so obtainedrepresent fairly the values which would be obtained when applying theconductivity test, as described hereinbefore, to the unwashed carrierand the washed carrier, respectively.

[0101] In testing the catalyst, the space-time-yield was kept constantat 750 g vinyl acetate/(l catalyst.h) by increasing the jackettemperature. The initial selectivity was 93%, and after 425 hours, thejacket temperature was 168° C., and the selectivity was 91%.

EXAMPLE 3

[0102] (According to the Invention, Prophetic)

[0103] A catalyst is prepared in accordance with the procedures ofExample 2, except for the following differences: step 3 is omitted, andfollowing step 4 the palladium/carrier composition is dried in air at120° C., then subjected to reduction in a hydrogen/nitrogen (20:80 v/v)mixture, at a flow rate of the hydrogen/nitrogen mixture of 500 Nl/(lcatalyst.h), at a temperature of 200° C., and at an absolute pressure of110 kPa (1.1 bar), during 4 hours.

EXAMPLE 4

[0104] (According to the Invention, Prophetic)

[0105] A catalyst is prepared in accordance with the procedures ofExample 2, except for the difference that step 3 is omitted, and thatinstead the palladium/carrier composition is dried in air at about 120°C., and cooled, and then subjected to reduction in a hydrogen/nitrogen(20:80 v/v) mixture, at a flow rate of the hydrogen/nitrogen mixture of500 Nl/l(l catalyst.h), at a temperature of 200° C., and at an absolutepressure of 110 kPa (1.1 bara), during 4 hours.

EXAMPLE 5

[0106] A catalyst was prepared from washed and dried carrier as obtainedin Example 2, by applying the following steps:

[0107] 1. A 25 g sample of the washed and dried carrier was impregnatedwith 15.7 ml of a solution of sodium palladium (II) tetrachloride (III)(Na₂PdCl₄) and tetrachloroauric acid (HAuCl₄) in de-ionized watercontaining 0.220 g palladium and 0.121 g gold. A container holding thecarrier was gently shaken to allow solution uptake by the carrier. Aftercomplete uptake of the solution the impregnated carrier was allowed tostand for 2 hours at room temperature.

[0108] 2. Then 30 ml of a solution containing 1.68 g sodium metasilicatepentahydrate (Na₂SiO₃.5H₂O) was added to completely cover the wetimpregnated support. This was allowed to stand for 15 hours.

[0109] 3. Subsequently, 2.5 ml. of 2.8% w hydrazine hydrate in water wasadded, mixed gently and allowed to stand for 4 hours at roomtemperature. This led to the reduction of about 55 mole-% of the totalof the palladium and gold salts to metallic species.

[0110] 4. The palladium/carrier composition was then washed withdistilled de-ionized water three times by decantation followed by acontinuous wash until the wash water was free of chloride, as checked bythe absence of a precipitate when tested with a silver nitrate solution.The washed palladium/carrier composition was then dried at about 120° C.for 4 hours under nitrogen, and cooled in a container protected frommoisture.

[0111] 5. The palladium/carrier composition was then subjected toreduction in a hydrogen/nitrogen (15:85 v/v) mixture, at a flow rate ofthe hydrogen/nitrogen mixture of 500 Nl/(l catalyst.h), at a temperatureof 220° C., and at a pressure of 100 kPa, until completion of thereduction, i.e. about 2 hours.

[0112] 6. The palladium/carrier composition was then impregnated with15.7 ml of a solution of potassium acetate in de-ionized watercontaining 1.34 g potassium, dried at about 120° C. for 15 hours undernitrogen, and cooled.

[0113] The catalyst thus prepared had a palladium content of 0.75% w, agold content of 0.4% w and its carbon monoxide chemisorption was 25.3mmol per kg catalyst.

EXAMPLE 6

[0114] (Prophetic)

[0115] Vinyl acetate is prepared as follows.

[0116] The catalyst prepared in Example 5 is tested in a reaction tubehaving a length of 30 cm and an inside diameter of 1.51 cm. The tube isloaded with 2.5 g of catalyst diluted into a 10 cm bed of glass beads.The reaction tube is fed with a gaseous mixture of 49 mole-% ethylene,13 mole-% acetic acid and 7.6 mole-% oxygen (balance nitrogen). The GHSVis 4250 Nl/(l.h), calculated on undiluted catalyst, and the pressure is880 kPa (i.e. 7.8 barg). With the catalyst temperature initially at 147°C., vinyl acetate is produced. The jacket temperature is increasedslowly so as to keep the space-time-yield constant.

EXAMPLE 7

[0117] (Prophetic)

[0118] Example 5 is repeated, with the exception that, in step 3, 2.5 mlof 12% w hydrazine hydrate in water is used instead of 2.5 ml of 2.8% whydrazine hydrate in water. This leads in this step 3 to the reductionof about 82-85 mole-% of the total of the palladium and gold salts tometallic species.

[0119] The catalyst so prepared is tested in the preparation of vinylacetate as described in Example 6.

1. A process for preparing a catalyst which process comprises the stepsof (a) selecting a carrier which is a silica based carrier which hasbeen subjected to a series of washings with one or more aqueous liquidsconsisting of aqueous liquids which have a pH of least 3, when measuredat 20° C., or which is a silica based carrier which is formed frommaterials one or more of which have been subjected to a said series ofwashings, (b) precipitating a Group 8 metal compound onto the carrier,(c) converting the precipitated Group 8 metal compound into metallicspecies, and (d) subjecting the Group 8 metal/carrier composition to apurification treatment, before or after step (c).
 2. A process asclaimed in claim 1, wherein the washing liquid is water.
 3. A process asclaimed in claim 1 or 2, wherein the extent of washing is such that in aconductivity test the conductivity measured for the washed carrier isless than 30% of the value found for the unwashed catalyst, wherein theconductivity test comprises contacting samples of water with samples ofthe unwashed carrier and the washed carrier and measuring at 95° C. theconductivity of each water sample after it has reached equilibrium withthe respective carrier sample at 95° C., the quantity of water samplebeing 3 g/g carrier sample, the conductivity of the water prior to thecontacting with a carrier sample being 1.5 μmho at 98° C., and theconductivities being measured by using a conductivity measuring probehaving a cell constant of 1.0/cm.
 4. A process as claimed in claim 3,wherein the extent of washing is such that in the conductivity test theconductivity measured for the washed carrier is less than 20% of thevalue found for the unwashed catalyst.
 5. A process as claimed in any ofclaims 1-4, wherein the extent of washing is such that in a conductivitytest the conductivity measured for the washed carrier is less than 75μmho, wherein the conductivity test comprises contacting a sample ofwater with a sample of the washed carrier and measuring at 95° C. theconductivity of the water sample after it has reached equilibrium withthe carrier sample at 95° C., the quantity of water sample. being 3 g/gcarrier sample, the conductivity of the water prior to the contactingwith the carrier sample being 1.5 μmho at 98° C., and the conductivitiesbeing measured using a conductivity measuring probe having a cellconstant of 1.0/cm.
 6. A process as claimed in claim 5, wherein theextent of washing is such that in the conductivity test the conductivitymeasured for the washed carrier is less than 50 μmho.
 7. A process asclaimed in any of claims 1-6, wherein the Group 8 metal and optionallyin addition a Group 1b metal is precipitated onto the carrier by poreimpregnating the carrier with one or more aqueous solutions comprising aGroup 8 metal compound precursor and optionally a Group 1b metalcompound precursor and then precipitating a Group 8 metal compound andoptionally a Group 1b metal compound onto the carrier from suchsolutions, by using a precipitating agent.
 8. A process as claimed inclaim 7, wherein the Group 8 metal compound precursors and the optionalGroup 1b metal compound precursors are selected from water soluble acidsand salts of palladium and optionally gold and the precipitating agentare selected from hydroxides, bicarbonates, carbonates and silicates oflithium, sodium and potassium.
 9. A process as claimed in any of claims1-8, wherein the precipitated Group 8 metal compound and, if present, aprecipitated Group 1b metal compound is converted into metallic speciesby reduction employing hydrogen as the reducing agent.
 10. A process asclaimed in any of claims 1-9, wherein the purification treatmentinvolves a series of washings with one or more aqueous liquids.
 11. Aprocess as claimed in any of claims 1-10, which comprises in addition astep of (e) impregnation with a source of an alkali metal.
 12. Acatalyst which is obtainable by a process for preparing a catalystaccording to any of claims 1-11.
 13. A catalyst as claimed in claim 12,which comprises palladium as the Group 8 metal in a quantity in therange of from 10 to 500 mmoles/kg catalyst, and in addition gold as aGroup 1b metal in a quantity in the range of from 1 to 200 mmoles/kgcatalyst, and an alkali metal in a quantity in the range of from 0.1 to5 mole/kg catalyst.
 14. A process for preparing an alkenyl carboxylatecomprising reacting a mixture comprising an olefin, a carboxylic acidand oxygen in the presence of a catalyst as claimed in claim 12 or 13.